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T4 bacteriophages on E.coli. Coloured Transmission Electron Micrograph (TEM) of T4 bacteriophage viruses attacking a bacterial cell of Escherichia coli. Dozens of viruses are seen, each with a head (capsid, yellow) and a tail. Bacteriophages infect only bacteria; T4 is a specific parasite of Escherichia coli. Bacteriophages occur in sewage, polluted water and faeces. T4 attaches itself to the cell wall of Escherichia coli with its tail. The elongated tail is a contractile sheath which contracts to squirt the contents of the head, the DNA genetic material, into the host cell. T4 DNA commandeers the genetic machinery of the cell, forcing it to reproduce more T4 bacteriophages.